Temperature Control

Seth McBride tries to cool off by sitting in a headwind while a friend provides shade.

In late summer I felt like I was baking myself as I pedaled my handcycle up a false flat. A thunderstorm gathered along the edge of the Cascade Mountains. The temperature was in the mid-80s, but the air was stagnant and thick with humidity.

The heat generated by my working muscles congregated in my core, and unable to escape, quickly built inside me. Ahead of me, my wife could not ride any slower and was disappearing into the distance. I rode at the limits of my steadily diminishing power, but my heart rate would barely rise.

Ten minutes later she turned off the road and I followed her to rest under the shade of an oak tree. We had ridden seven miles and I could ride no more. After 20 minutes I started to feel more functional, but I was despondent. We were set to leave on a very long bike trip in the fall, cycling south for thousands of miles. How was I supposed to ride through Mexico if I couldn’t even cover seven miles on a summer day in Washington?

What Science Tells Us About Our Bodies’ Heating and Cooling Systems

Anyone with a neurological disability knows what a problem overheating can be. If you have a spinal cord injury or multiple sclerosis in particular, chances are the heat affects you far more than anyone you know without a disability. This was definitely true for me. What I didn’t know is why, exactly, my body struggled so much, and what I could do about it. Fortunately, recent research and practical experience helps to shed some light on what we all can do to stay functional in the heat.

Let’s start with spinal cord injuries. If you have an SCI, you have issues sweating. Some people sweat above the level of their injury, some sweat some below, some not at all. Some sweat when they’re hot, some only sweat when they’re getting dysreflexic. Like the rest of your neurological function, this is all related to your level of injury and its completeness.

T6 is a big dividing line for nervous system function. Injuries above T6 generally have a compromised sympathetic nervous system. Somewhere around T4 (depending on the individual and completeness of injury), people lose the ability to sweat, even above their injury. This is obviously an issue for temperature regulation, as the evaporative cooling of sweat is one of our body’s primary means of cooling itself.
But people with injuries above T6 also have issues with heart rate and circulation. We (I have a C7-8 mostly complete injury) can’t get our heart rates as high as lower level injuries, or the nondisabled. Both muscles and organs generate internal heat. The body cools itself by shuttling that heat away from the core of the body via circulating blood to the skin, where it can be effectively dissipated.

When someone with a high level injury is sitting around (lounging by the pool, in a perfect world), we have ample capacity to shuttle heat from our core to our skin. As soon as we start working — pushing, handcycling, impromptu dance battling, you name it — our bodies’ compromised circulatory capacities come into effect. The heart’s primary responsibility is to maintain blood flow to the working muscles, and because we can’t get our heart rate high enough to do much else, its other responsibilities get neglected. The heat we generate stays in our core, effectively baking ourselves from the inside. Metaphorically speaking, we are both the oven and the roast.

If you can sweat, it obviously does help cool you — it’s one of the body’s primary means of shedding heat. But sweating is only effective relative to the surface area that’s actually perspiring. If you only produced sweat on your left forearm, it wouldn’t be very effective for cooling off your whole body. So the higher your level of injury, the less cooling benefit you get from sweat — since people typically only sweat above their injury.

With all this data, researchers now consider temperature regulation in people with SCI to be proportional to their level of injury. In short, the higher your injury, the more you’re going to have issues with heat (and cold). Nondisabled people or those with low paraplegia have a certain amount of temperature rise in hot weather or when they’re working hard, but that temperature quickly settles out to an elevated but manageable degree. Because higher level injuries can’t circulate and dissipate heat effectively, our temperatures rise … and then keep rising.

How Hot Is Too Hot?

People with multiple sclerosis don’t have any of the same issues with heat retention that those with high-level spinal cord injuries do. For those with MS, small changes in core temperatures — even what’s caused by taking a hot shower —can have significant impact on their disease symptoms. Until recently, the test for determining if someone had MS was for doctors to measure their neurologic state after they’d taken a warm bath.

Mona Sen is a trained occupational therapist, author and activist who has multiple sclerosis. She now lives in upstate New York, where it’s relatively easy to manage the heat. At her house, it rarely gets too hot, but she still has to be careful: only going outside for short stretches during a warm summer day; occasionally wearing an ice vest; and always having an exit plan for the heat. She knows too well how much getting overheated can affect her.

“To me, heat is like kryptonite is to Superman,” she says. “I just get weak … almost like the demyelination (the disease process by which MS causes neurologic problems) in my body kicks in, and it’s hard to move. It’s hard to walk at all … and my mind, it just kind of slows down.”

Fortunately, the degradation in function is very temporary, often disappearing as soon as you start to cool off. For Sen, this change in function can often be very dramatic.

“I was in North Carolina last summer and it was extremely hot, but my friend has central air in the house and it was fine,” she says. “But my friends decided they wanted to go on this walk. I said nah, I think I’m going to stay in, I know how this goes. But they talked me into it. And as this walk proceeded, I got hotter, and we finally got back to the car and they had to literally help me out of my rollator (a rolling walker) and my speech started to slur because I was so hot. I got in the car, and the air was on full blast for about three to five minutes. My speech totally came back. Everybody just sat there and their jaws dropped. That’s how it works.”

For those with SCI, it takes a much bigger rise in temperature before we start to feel the effects. Usually our core temp has to rise to about 102 degrees or above, depending on the individual. Unfortunately, because there’s such a large rise, it can take much longer to cool off again.

Joe Delagrave is an athlete with C6-7 incomplete quadriplegia who lives in Phoenix, Arizona. He has learned to live in an environment of extreme heat. Even so, Delagrave says that he’s had the beginning stages of heat illness a number of times since he moved to Phoenix seven years ago. He says, “It happens with a combination of things — dehydration, exhaustion, just not being prepared for the heat.”

When it does happen, heat illness can be debilitating and prolonged. “It kind of just zaps my energy, where I’m useless for the rest of the day. Lethargic,” he says. “It usually takes a handful of hours, if I get really overheated in the early afternoon, either working out, or just being out running a bunch of errands, and I can’t get into a cold shower. It can be 6 or 7 o’clock before I’ve actually cooled off.”

What To Do

Obviously, preventing overheating is both a challenge, and extremely important if you have a neurological disability. In addition to heat related illnesses, for some getting too hot (or too cold) can even precipitate autonomic dysreflexia. Dramatic changes in temperature are like any other noxious stimuli. To get a better handle on the science and best practices for staying functional in the heat, I talked with Elizabeth Broad, a dietician and sports scientist who currently works for U.S. Paralympics. She has decades of experience working with athletes with a variety of disabilities, and to her work, cooling strategies are of vital importance. “Overheating can be a big deterrent to being active outside for people with disabilities,” she says. “If we can work out practical ways to deal with this, then hopefully we get more people exercising!”

Broad says one of the simplest strategies for keeping cool in the heat is something that gimps have been doing for a long time: misting themselves with a spray bottle filled with cold water. When you spray yourself, the area of your skin that the spray touches is effectively mimicking the cooling effects of sweat. But like sweat, spray is effective relative to the surface area that’s actually wet. So spraying your face feels good, but spraying your face, neck, shoulders, arms, and whatever other skin is exposed, is going to do a better job of cooling you off.

Spray works best when humidity is low because evaporation rates are higher. When humidity is high, the water just sits on your skin and doesn’t actually cool you. If it’s really hot, or if humidity is high, combining a spray bottle with either a natural breeze or a fan is the best way to stay cool.

Paralympic handcyclists, Broad says, will fill leg stockings with ice and put them under the jersey, on the abdomen to help keep core temps from rising while they warm up for a race. Trainers will also put towels in a bucket of ice water, wring them out a bit and then put them over athletes’ heads or necks.

Things like ice vests, cold wet towels, or a gel pack that you keep in the freezer and then drape around your neck feel great when you first put them on. But if you don’t keep rotating them or have air circulating between their external surface and your skin, the heat from your skin warms up the outermost layer after a minute or two, creating its own little microclimate and stopping the cooling process. The solution to this is to either have multiple towels or neck wraps that you can rotate, or to get a breeze blowing between the external cooling source and your skin.

Research has also shown that simply placing a hand in a bowl of cool water (about 50 degrees, or cold out of the tap) for 10 minutes can lower core temperatures by one degree Fahrenheit, which may not seem like a lot, but could be the difference between being worthless and somewhat functional.

Precooling, or purposefully lowering your core temp before going out in the heat, can also be an effective strategy if you know you’re going to be active in the heat for a long period of time. Most research studies have used ice baths as a method for lowering core temperature, which isn’t exactly the most practical way to cool yourself if you have an SCI. But taking a cold shower, drinking an ice slushy or even sitting in an overly air-conditioned room for long enough that you start shivering — all are effective ways of lowering your core temperature.

Combining precooling with other cooling strategies — spray, fans, ice stockings or towels — is the most effective way to stay functional in the heat for a long period of time.

Delagrave doesn’t typically do any precooling, but uses a cold shower as a way of quickly cooling down his core temp if he gets overheated. He does his outdoor workouts early in the morning and carries a spray bottle around with him, even when he’s running errands in the summer. He even decreases the amount of his bladder medication, Oxybutynin, in the summer because one of its side effects is increased heat intolerance. In addition, he makes sure to stay hydrated. “I probably drink twice the amount of water in the summer that I do in the winter.”

Testing the Limits

After my ill-fated summer training ride, I had a friend design a spray system for my handcycle that included a water tank of PVC pipe equipped with a tire valve at one end and a line of automotive hosing running out the other. The hose ran to a release valve that I zip-tied to the seat of my handcycle and could operate with one hand before continuing along to a spray nozzle pointed at my face and upper body. I could fill the tank with cold water, pressurize it with a bike pump, and then spray myself with a quick tap of the valve while still on the move. Quadgrips is a company now selling a similar system — the QuadCool.

Not knowing much about the science of cooling, when I first tried it, I was amazed at how well it worked. I probably shouldn’t have been. It allowed me to easily spray a relatively large surface area of my upper body while on the move, and the breeze generated by my motion helped quickly evaporate the water, drawing heat out of my skin. It instantly increased the temperature in which I could comfortably ride by 10-15 degrees.

In the process of trying to ride a handcycle through the tropics, I learned a number of things about the limits of my body and my temperature regulation abilities. I learned that I could ride long distances in 90-degree desert temperatures if I consistently sprayed myself and stayed on top of my hydration. I learned that it’s possible, if painful and requiring many more rest breaks, to climb hills in the same conditions. Riding long stretches in 90-degree humid temperatures was technically possible, but right at the limits of my body’s ability to shed heat. To add significant hills to the same conditions put my body over the top.

You may never want to try to ride a handcycle through Mexico. But if you struggle in the heat, wheeling around town on a particularly hot summer day can feel like just as monumental a task. Like anything else with a disability, learning how to better function in the heat comes down to being willing to try some different strategies, adapt them, and see what works best for you. Fortunately, some simple solutions can keep you active all year long.